In vehicles that run on gasoline, ignition of the fuel/oxidant mixture in each cylinder is commanded by a spark plug (in a diesel engine, the fuel/oxidant mixture self-ignites when it reaches a predefined self-ignition temperature) and has to occur at a suitable point in the cycle in order to obtain optimum efficiency in the combustion of the mixture. When ignition occurs too early, vibrations occur and may damage the engine. When ignition occurs too late, the efficiency of the cylinder decreases significantly. Optimum efficiency is obtained for ignition occurring in a predefined time window, in which vibrations are extremely low at the start of the window and near-zero at the end of the window. In these vehicles, self-ignition is to be avoided in order to guarantee that ignition occurs using the spark from the spark plug within the predefined time window. Ignition that occurs at an inopportune time in the cycle may also damage the cylinder or the piston by creating overpressure within the combustion chamber in the wrong part of the stroke.
Another phenomenon liable to damage cylinder and piston (in gasoline or diesel engines) is knock. Knock is disordered combustion, with the appearance of erratic zones in which one or several combustions begin spontaneously before coming into contact with the flame front, leading to pressure spikes and to the explosion resonating off the walls of the combustion chamber and of the piston. These pressure spikes are very damaging to the piston and the cylinder liners of the engine, and also to the cylinder head gasket and the spark plug.
In order to ignite the mixture in each cylinder during the optimum time window or to adjust the fuel/oxidant mixture in order to prevent or reduce knock, it is known practice to monitor the vibrations of each cylinder using one or more knock sensors
Such a knock sensor conventionally comprises:                a body comprising:                    a metal mount notably having a central bore to accept a fixing screw so that the knock sensor can be fixed to the engine block (the torque applied to this fixing screw is of the order of 20 N.m) and a lower support ring to accept a detection assembly,            said detection assembly, which usually comprises a piezoelectric element between two contacts surmounted by a seismic mass for amplifying the signal,                        an overmolded peripheral casing to protect the detection assembly, made from a thermoplastic material applied by overmolding at least in part around the body.        
It should be noted that, throughout the description, any knock sensor is observed in a position in which the central axis of the mount is vertical and its support ring is toward the lower part. The terms and expressions “lower”, “upper”, “below”, “above”, “top”, “bottom”, etc., are relative to this viewpoint.
In order for the peripheral casing to be able to perform its function of protecting the detection assembly, it is absolutely essential for contact between the casing and the mount to be close contact, and for this to be true at all times. Now, a gap may arise between the mount and the peripheral casing, into which water can infiltrate, leading to potential damage to the detection assembly. This gap is a result on the one hand of the overmolding method which is unable to ensure total adhesion of the casing to the mount and results in the appearance of a manufacturing clearance between the mount and the casing. The aforementioned gap is also the result of the high temperatures to which the knock sensor is subjected in use: an expansion clearance combines, in operation, with the manufacturing clearance because of the differential expansion of the mount, which is made of metal, with respect to the peripheral casing, made of plastic (the two materials present having different thermal expansion coefficients).
In order to limit the risks of water infiltrating between the mount and the peripheral casing, it is known practice to use a mount which, on a peripheral face of the lower support ring, has grooves to accept thermoplastic material so as to improve the attachment of the thermoplastic casing to the metal mount and, as a result, the sealing of the junction between mount and casing.
Nevertheless, the contact between the casing and the connecting mount is not close enough and defective sealing between the overmolded peripheral casing and the metal mount is still observed far too often. The expansion of the metal mount occurs preferentially along an axis perpendicular to a central axis which corresponds to the axis of the central bore.